Re: COPY FREEZE and setting PD_ALL_VISIBLE/visibility map bits - Mailing list pgsql-hackers

On 12.01.2021 22:30, Tomas Vondra wrote:
> Thanks. These patches seem to resolve the TOAST table issue, freezing 
> it as expected. I think the code duplication is not an issue, but I 
> wonder why heap_insert uses this condition:
>
>     /*
>      * ...
>      *
>      * No need to update the visibilitymap if it had all_frozen bit set
>      * before this insertion.
>      */
>     if (all_frozen_set && ((vmstatus & VISIBILITYMAP_ALL_FROZEN) == 0))
>
> while heap_multi_insert only does this:
>
>     if (all_frozen_set) { ... }
>
> I haven't looked at the details, but shouldn't both do the same thing?


I decided to add this check for heap_insert() to avoid unneeded calls of 
visibilitymap_set(). If we insert tuples one by one, we can only call 
this once per page.
In my understanding, heap_multi_insert() inserts tuples in batches, so 
it doesn't need this optimization.

>
>
> However, I've also repeated the test counting all-frozen pages in both 
> the main table and TOAST table, and I get this:
>
> patched
> =======
>
> select count(*) from pg_visibility((select reltoastrelid from pg_class 
> where relname = 't'));
>
>  count
> --------
>  100002
> (1 row)
>
>
> select count(*) from pg_visibility((select reltoastrelid from pg_class 
> where relname = 't')) where not all_visible;
>
>  count
> --------
>       0
> (1 row)
>
> That is - all TOAST pages are frozen (as expected, which is good). But 
> now there are 100002 pages, not just 100000 pages. That is, we're now 
> creating 2 extra pages, for some reason. I recall Pavan reported 
> similar issue with every 32768-th page not being properly filled, but 
> I'm not sure if that's the same issue.
>
>
> regards
>

As Pavan correctly figured it out before the problem is that 
RelationGetBufferForTuple() moves to the next page, losing free space in 
the block:

 > ... I see that a relcache invalidation arrives
 > after 1st and then after every 32672th block is filled. That clears the
 > rel->rd_smgr field and we lose the information about the saved target
 > block. The code then moves to extend the relation again and thus 
skips the
 > previously less-than-half filled block, losing the free space in that 
block.

The reason of this cache invalidation is vm_extend() call, which happens 
every 32762 blocks.

RelationGetBufferForTuple() tries to use the last page, but for some 
reason this code is under 'use_fsm' check. And COPY FROM doesn't use fsm 
(see TABLE_INSERT_SKIP_FSM).


         /*
          * If the FSM knows nothing of the rel, try the last page before we
          * give up and extend.  This avoids one-tuple-per-page syndrome 
during
          * bootstrapping or in a recently-started system.
          */
         if (targetBlock == InvalidBlockNumber)
         {
             BlockNumber nblocks = RelationGetNumberOfBlocks(relation);
             if (nblocks > 0)
                 targetBlock = nblocks - 1;
         }


I think we can use this code without regard to 'use_fsm'. With this 
change, the number of toast rel pages is correct. The patch is attached.

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company